Should work now even if homebrew is installed in non-standard location

I am the sort of [insert negative word here] who installs his homebrew into a non-standard location.  The gradle files for beaker notebook assume that everything installed via homebrew is in the normal /usr/local location.  This patch is an attempt to fix this.  It works on my machine.

Caveat : I have never written any Groovy or Gradle before.